To perform effective noise control, it is essential that the detailed character of the noise source be well understood. Near-field Acoustical Holography (NAH) is a powerful tool for visualizing noise sources and thus for identifying the detailed nature of a source. NAH has been applied most often for visualizing structure-borne and machinery noise sources. In this book, it is shown how NAH can be applied with equal success to aeroacoustic sources. A detailed development of multireference NAH is provided to allow the use of a scanning microphone array. Also, methods for compensating for source nonstationarity and for extrapolating sound fields beyond the measurement aperture to improve the accuracy of the source imaging are described. Experimental results are given for a ducted fan and for a free jet, and it is shown, for example, that nearfield measurements allow the accurate prediction of a source’s farfield directivity. The analysis presented in this book will help scientists and engineers concerned with visualizing aeroacoustic sources, in particular, and more generally, any practical machinery noise sources that comprise several uncorrelated or partially correlated subsources.

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